The present invention relates to a safety switching device for shutting down a technical installation in a failsafe manner and to a method for testing inputs of such a safety switching device.
In general, the invention relates to the field of machinery safety in terms of protecting persons and/or material valuables from dangers that are posed by technical installations or machines that function automatic manner, such as for example fully automated robots. In order to make installations of this type safe, different signaling devices, such as for example emergency OFF switches, position switches or electro-sensitive protective devices such as light barriers and light arrays can be used, with which it is possible to monitor access to an installation and to monitor that the installation is functioning in a safe manner. The signaling devices are coupled to a safety switching device that in dependence upon the signaling devices can transfer the technical installation into a safe non-critical state, in particular into a state in which the installation is shut down. It is therefore of great importance that the state transmitted by a signaling device is correctly identified by the safety switching device, in particular the OFF-state, in which the installation is to be shut down.
In order to enable a fail-safe shut down of the technical installation in the event of an incorrect input, the signaling devices are frequently coupled to the safety switching device in a two-channel, redundant manner by way of two inputs. The redundancy ensures that the installation is shut down even in the event of an error occurring in one of the channels. This procedure prevents the effects of a critical error but does not immediately identifies the critical error In other words the safe functioning of the installation is ensured but the error in one of the channels itself is not discovered. In order not only to prevent the effects of an error but also to identify the error, the inputs of the safety switching devices are equipped with self-test functions, with which said safety switching devices are able to test the detection capability of an individual input. Input circuits of this type are for instance known from DE 10 2006 030 114 B4 or from DE 10 2011 015 498 A1.
DE 10 2006 030 114 B4 discloses a prior art safety switching device with an input circuit for reading in input signals from a signaling device. The safety switching device comprises a signal input circuit and an evaluation circuit that can be switched on by a coupling member. The coupling member is connected to a reference voltage source to set a switching threshold that is to be exceeded by the input signal. Furthermore, the input circuit comprises a first testing circuit for testing the reference voltage source, and a second testing circuit with which a voltage can be injected into the signal input circuit in order to test the coupling member. The illustrated input circuit is however disadvantageous in that the testing circuits have in each case one or two coupling elements that are galvanically separated from one another in order to provide a galvanic separation between the signal input circuit and a subsequent logic unit initiating the tests. Coupling elements of this type are expensive in comparison to other components of the inputs and they require a large amount of installation space and they have a high failure rate.
DE 10 2011 015 498 A1 discloses a further prior art safety switching device. The safety switching device comprises an input circuit with a signal input circuit and an evaluating circuit that can be switched on by a coupling member. An additional transistor circuit is arranged before the coupling member so that a safe state can be detected either by the current of the input signal or by the voltage of the input signal. The switching threshold is set by a voltage reference. As is the case in DE 10 2006 030 114 B4, two testing circuits are provided. A first testing circuit tests the coupling member and the transistor, and the second testing circuit is used to test the reference voltage. The first and the second testing circuit comprise coupling elements for providing the galvanic separation between the signal input circuit and the logic unit that is connected subsequent thereof. In total, at least three coupling elements are required for the test devices. An advantage of the illustrated input circuit is that only one reference voltage source is required and thus only one testing circuit for said reference voltage source is needed. However, it is disadvantageous that all the parameters that are used to set the switching threshold are not fully checked by means of the second test circuit since the switching threshold in the illustrated circuit is not only dependent upon the voltage reference but also depends on the parameters of other components, in particular the parameters of the transistor circuit that is connected upstream thereof. It is thus desired to provide a safety switching device with enhanced testing capabilities in a cost-effective manner.